Forming and casting of semi-solid metal, and like thixotropic materials is an important technology that has a growing number of applications, such as in motor vehicles such as in engine mounts, suspension brackets, and cylinder housings. While excellent reproducibility, lower temperature operation, and near net shape processing are the principle advantages of this technology, for certain low tolerance, high quality applications, inclusions remain a problem.
Generally thixotropic material forming and casting involves loading a billet into a piston chamber, and thrusting a piston into the piston chamber to propel the billet through an injector into one or more mold cavities. Lubricants are often used to facilitate reciprocation of the piston (or like element) by reducing resistance to the motion from the billet and piston bearing against the chamber. Depending on the billet and the environmental conditions, a skin of varying thickness, mechanical properties, and having various layers are typically formed. The layers of the skin may have respective impurities, and may pose problems for uniform filling of the molds. In horizontal injection systems, the piston and chamber containing the injection matter (often called a sleeve) are oriented so that the piston moves horizontally. The lubricant used on the piston and container walls may accumulate on the bottom of the container.
Inclusion-type defects, due to the skin or layers thereof being entrained in the flow, are some of the major problems associated with forming processes where the injector includes a throttling neck. When the matter is being injected, the skin layers can get into the parts or finished products, where they remain trapped. The skin may have lubricant or other impurities that lead to inclusions. Inclusions are one of the major defects found in formed parts. They can significantly reduce the quality and mechanical properties of the parts.
The lubricants in particular can aggravate the inclusion problem by contaminating surfaces. Lubricant traces get into the parts or finished products along with the outside layers. When a part containing lubricant is heated, the lubricant may decompose and create oval or half-moon porosities referred to as lenticular porosities. Heat treatment is often used on parts made using semi-solid metal casting to increase their mechanical properties.
Another phenomenon occurring during injection, under the effect of heat transfer, is that the outside of the injected matter is rapidly cooled by contact, thus creating a relatively hard, or pre-solidified, layer.
One solution well known in the art is to provide an oxide trap. The most commonly deployed oxide traps are annular skimming ring tanks that surround the neck radially, and use an edge at the base of the throttling neck to direct a skin of the billet radially into the ring tank, such that it is excluded from the neck, and so is not delivered to the mold. Applicant has found that using such annular skimming tanks, presolidified skin backs up in a dead-zone forming a ramp that actually encourages skin to be entrained into the neck, to the detriment of the forming process.
U.S. Pat. No. 5,730,201 to Rollin et al. teaches an improved oxide trap. According to the teachings of Rollin et al. before introducing the thixotropic metal alloy into mold cavity, the oxide skin surrounding the metal billet is removed completely and collected in an oxide deposit ring. Rollin et al. strives to minimize the removal of oxide-free, homogeneous thixotropic metal alloy, by taking into account the thermal and mechanical properties of the thixotropic billet, which are asymmetric with respect to the longitudinal axis of the metal billet. While it is stated that an essentially uniformly thick oxide skin is formed over the whole of the thixotropic metal billet, it makes mechanical and thermal contact with the casting chamber wall over only a small area on its undermost side. More heat is lost by the billet there. Rollin et al. reasons that optimal removal of the oxide skin requires accounting for asymmetric (with respect to the longitudinal axis of the metal billet) thermal and mechanical properties of the oxide skin. According to Rollin et al., the thixotropic metal alloy is led through a ring-shaped body situated between the casting chamber and the mold. It is stated to be essential to the invention that while removing a constant amount of oxide over the whole peripheral region of the billet, the oxide remover opening features a cross-section that is asymmetric with respect to the concentric middle axis of the oxide remover, such that the lower part of the horizontal ring-shaped oxide remover is larger than that of the upper part.
Rollin et al. also teach a recess 44, preferably situated in the lower part of the oxide remover opening 42, arranged to enlarge a cross-section of the opening in this lower region, and serves to provide better guide the oxide skin into the corresponding part of the oxide deposit ring 40. The preferred example of the recess has an angular extent of 65° with respect to the axis.
While Rollin et al. claim that shaped parts manufactured by their process typically exhibit a porosity of less than 1 vol. % and an oxide fraction of 0-3 wt. %, and preferably 0-1 wt. %, the requirement to remove an annular sheath is expensive and wasteful. The method according to Rollin et al. produces a thickened oxide layer, and superheats the skin (inevitable given induction heating), which leads to less uniform billets. There remains a need for an effective oxide remover for thixotropic material injectors around a throttling neck.